The invention relates to a TFT (thin film transistor) active matrix for an optical sensor, of the type which comprises:
a substrate;
a matrix of TFT transistors formed on this substrate, each transistor comprising a gate, a source and a drain;
a set of row lines for controlling the TFT transistors by means of their gates, this set of row lines being placed on the substrate;
a conducting level in a particular pattern forming a matrix of electrodes, each electrode defining a zone called a pixel; these electrodes being connected to the drains of the transistors;
a set of column lines making it possible to transfer charge through the transistors between the electrodes and external electronics, the column lines being connected to the sources of the TFT transistors;
an insulating layer between the electrodes and the column lines, this insulating layer being open locally over each pixel in order to put the pixel electrode in contact with the drain of the TFT transistor;
a photosensitive semiconducting layer in contact with the pixel electrodes in order to convert the electromagnetic radiation into electric charges collected by the electrodes.
An active matrix of this type can be used in an optical sensor. The electromagnetic radiation which strikes the photosensitive semiconducting layer is converted into electric charges which are collected by the pixel electrodes. These electric charges are processed in an electronic circuit in order to reconstruct an image point by point, for example using liquid crystal displays (LCDs). One particular use of the TFT active matrix relates to producing images from X-rays, the semiconducting layer advantageously being made of selenium.
U.S. Pat. No. 5,780,871 shows an active matrix of this type having a high pixel aperture ratio which, however, leads to appreciable capacitive coupling between pixel electrodes and row lines, despite an insulating layer provided to counter this coupling; such capacitive coupling is disadvantageous to the quality of image desired.
A first aim of the invention is to improve the image reconstructed from an optical sensor equipped with a TFT active matrix as defined above.
To do this, according to a first provision of the invention, a pixel electrode is located completely inside an outline defined by two successive row lines and two successive column lines, a clearance being provided between the inner edge of this outline and the periphery of the pixel such that the pixel electrode covers neither the row lines nor the column lines.
Under these conditions, the capacitive coupling between pixel electrode and row and column lines is considerably reduced, which prevents undesirable charges leading to image degradation.
Decreasing the optical aperture, which results from decreasing the useful area of the pixel, does not lead to an appreciable decrease in charge recovery nor to a decrease in image quality.
This result seems to be due to the kinetic behavior of the carriers generated in a photosensitive semiconductor by the electromagnetic radiation. Because of a lateral field created by the carriers, the latter naturally move toward the pixel electrodes.
Thus, according to the invention, reducing the capacitive coupling obtained by decreasing the dimensions of the pixel electrode does not lead to problems with regard to charge recovery. The restored image quality is improved.
Preferably, the mean clearance between the outline of the pixel electrode and the external outline formed by the row and column lines is substantially equal to twice the alignment tolerance of a photorepeater which makes it possible to produce the images of various patterns and designs of circuits and electrodes on the resist of the plate.
The mean value of the clearance may be between 4 and 8 xcexcm, preferably about 6 xcexcm.
Advantageously, in the matrix defined above, the TFT transistor associated with each pixel is covered by the pixel electrode, and is thus protected.
The insulating layer provided between the electrodes and the column lines is advantageously formed by a photoresist or photoimageable resist.
According to a second aim of the invention, it is desired to improve the contrast of the images obtained, by having a high capacitance over the pixels, in order to store a lot of charge. This is because, the more charge is stored, the better the image contrast.
However, the embodiment should remain simple and the manufacturing yield should remain high, especially by avoiding the short-circuit faults due to impurities or to dust as far as possible.
According to a second provision of the invention, which may be used independently or in combination with the previous provision, the matrix comprises:
a set of storage row lines forming capacitors with the pixel electrodes or drains, these storage row lines being parallel to the control row lines,
and connections parallel to the column lines, forming bridges, provided in order to electrically connect the successive storage row lines and to make it possible, when controlling a row of pixels, to distribute the removal of charges from the entire row of pixels over several parallel storage row lines.
Preferably, the connections parallel to the column lines are at the same level as the column lines.
According to a first possibility, the storage row lines are provided in a level located on the substrate below the level of the control row lines; the storage row lines form a capacitor with the pixel electrodes or drains, or with a specific electrode made in the same level as the control row lines, and a thin insulating level is located between the level of the storage row lines and the level of the control row lines.
According to a second possibility, the storage row lines are substantially at the same level as the control row lines.
The number of connections parallel to the column lines may be equal to the number of column lines, that is one bridge per pixel. It is also possible to provide a number of parallel connections which is less than the number of column lines and therefore a number of bridges which is less than the number of pixels. In particular, a connection parallel to the column lines may be provided every sixteen column lines.
The high-capacitance zones defined by the stack of three layers, that is xe2x80x9cstorage row line levelxe2x80x9d, xe2x80x9cthin insulatorxe2x80x9d, and xe2x80x9cupper conducting levelxe2x80x9d, define planar capacitors in which the thin insulator is never used alone in order to electrically separate a step of the xe2x80x9cstorage row line levelxe2x80x9d from an xe2x80x9cupper conducting levelxe2x80x9d. This makes it possible to improve the manufacturing yield.
Preferably, the pixel comprises a device for protecting against excessively high voltages, which is incorporated into the transistor, when working at negative voltage, or comprising a specific transistor and a diode when working at positive voltage.
The invention also relates to an optical sensor, in particular for X-rays, equipped with an active matrix as defined above.